The present invention concerns the problem of correctly tensioning the thread used in a sewing machine.
Very often in sewing machines, when the user swings up the presser-foot lever of the machine, to raise the cloth presser foot from cloth-engaging to inoperative position, the machine's thread-tension disks are relieved, to terminate tensioning action. This is done so that thread can be freely drawn from the thread spool in the course of manually repositioning the workpiece for continued sewing, or as an incident to removal of the workpiece from the machine. In some machines, there is a complete loss of thread tension when the cloth presser foot is raised to inoperative position. However, this can be very disadvantageous for the case where basting work is done with the pressure foot kept raised, because a situation of insufficient thread tension can readily occur. In some machines, of the type where the thread-tensioning action is terminated upon raising of the presser foot, the mechanism employed is such that the thread-tensioning action happens to be progressively decreased in the course of raising the presser foot to inoperative position. A disadvantage with mechanisms of that type is that, when sewing a very thick workpiece, e.g., consisting of several layers of thick cloth, the presser foot may be kept elevated from its lowermost position by the thick workpiece itself, resulting in an incidental and inappropriate thread-tension decrease. Some sewing machines maintain non-zero upper thread tension, even when the cloth presser foot is raised to inoperative position, and this is potentially useful for basting work performed with the presser foot elevated. However, beyond the mere fact of non-zero upper thread tension in such a situation, it would really be necessary to be able to finely adjust the thread tension value, dependent upon the particular basting job involved, but this has been very difficult to provide and in general when the cloth presser foot is in raised, inoperative position, no fine-adjustment capability remains for thread-tension control.
Problems of appropriate thread-tension action can be particularly troublesome in the case of sewing machines having stitch-pattern capabilities, e.g., machines able to implement stitch-patterns of intricate configuration by resort to the now familiar pattern-ROM technique. In such patterns, the needle bar of the machine may be transversely displaced proceeding from one constituent needle-penetration location of a stitch-pattern to the next by transverse distances varying greatly from one constituent stitch of the pattern to the next, and likewise the amount and the direction of the cloth feed increment may vary from one needle-penetration action to the next with a considerable frequency and range of variation within the course of a single stitch-pattern. Furthermore, some stitch-patterns may be constituted by intervals of intricate stitch configuration alternating with intervals of simple straight stitching, further adding to the complexity of the tension-control aspect of pattern implementation. In general, the user of the machine is able to manually adjust thread tension, e.g., prior to commencement of automatic sewing of a particular stitch-pattern, but it is not in general feasible to manually adjust thread tension during the course of implementation of the successive stitches of such a stitch-pattern.
In addition to the tension-control implications of an intricate stitch-pattern geometry, the user must, of course, furthermore take into account factors of workpiece thickness and the thickness and character of the thread employed, e.g., slick thread or rough thread in the case of disk-type tensioning mechanisms which establish thread tension by mere frictional drag on the thread. These various factors can make it far from self-evident to the operator how the thread tension should be set for a particular job.